1. Field of the Invention
The present invention relates to an alloy tool steel, more specifically relates to an alloy tool steel which is applied to various tools including hot plastic working dies and warm plastic working dies.
2. Description of Related Art
Conventionally, for hot/warm plastic working dies used under severe conditions (i.e., at high bearing pressures and high temperatures), W—Cr series hot die steels such as JIS-SKD8, which places importance on softening resistance, and forging die steels such as JIS-SKT4 are applied.
In addition, in some fields of hot/warm working, a matrix high-speed tool steel is also used since high strength can be secured both at room temperatures and high temperatures. For example, Japanese Patent Application Unexamined Publications Nos. Hei2-8347 and Hei2-11736 disclose a hot working tool steel containing from 0.98 wt % to 1.61 wt % W and from 1.98 wt % to 2.55 wt % Mo in addition to elements such as C, Si, Mn, Cr, V and Co (see column 8, lines 1 to 14 and Table 1 in both the references). In these references, it is described that when one or both of W and Mo constitute from 2.0 wt % to 3.50 wt % in terms of 1/2W+Mo, softening resistance and high-temperature strength may be improved without deteriorating toughness.
Further, Japanese Patent Application Unexamined Publication No. Hei7-207414 discloses a steel for aluminum casting molds containing from 0.42 wt % to 3.21 wt % W and from 0.35 wt % to 3.70 wt % Mo in addition to the elements such as C, Si, Mn, Cr, V and Co (see paragraph 0014 and Table 1). In this reference, it is described that when one or both of W and Mo constitute from 0.2 wt % to 4.0 wt % in terms of 1/2W+Mo, deterioration in toughness may be prevented.
Furthermore, Japanese Patent Application Unexamined Publication No. 2003-268499 discloses, instead of the matrix high-speed tool steel, a high-speed tool steel in which from 2.0 wt % to 9.5 wt % Mo and from 1.0 wt % to 16.0 wt % W are contained in addition to the elements such as C, Si, Mn, Cr, V and Co, and 2Mo+W constitutes from 5 wt % to 20 wt % (see paragraph 0034). In this reference, it is described that when 2Mo/(2Mo+W) constitutes not more than 0.9, softening resistance at high temperatures may be improved and hardness against heat treatment may be secured during low temperature hardening.
JIS-SKD61 which is most widely used as the hot die steel contains Mo instead of W, so that it has improved toughness and is stable toward thermal shock. However, as SKD61 has the maximum hardness on the order of 50 HRC and softening resistance is insufficient, it cannot be applied to hot/warm plastic working dies used under severe conditions.
On the other hand, among JIS steels, the W—Cr series hot die steel (for example, SKD8) shows high strength and high softening resistance. However, even the W—Cr series hot die steel has the maximum hardness on the order of 55 HRC, and it is difficult to apply to dies for press forging used under severe conditions. In addition, the forging die steel (for example, SKT4) has problems that (1) application to a large product is difficult due to its insufficient hardenability, and (2) the maximum hardness is on the order of 55 HRC, thus high strength is not secured.
To the contrary, in the matrix high-speed tool steel, high strength may be secured and it is superior to the conventional hot die steel and forging die steel. However, as the conventional matrix high-speed tool steel is still insufficient in strength at 700° C., there is a problem that sufficient durability cannot be attained when it is applied in hot/warm plastic working during which an outermost layer reaches the order of 700° C.